The question, "'''What good is half a wing?'''" is often used by [[creationist]]s who severely misunderstand [[evolutionary theory]]. The question implies that [[fossil]] [[bird]]s should be discovered with literal "half-wings" &mdash; i.e., a wing missing half of itself. This is a misunderstanding of how wings likely evolved.

The question, "'''What good is half a wing?'''" is often used by [[creationist]]s who severely misunderstand [[evolutionary theory]]. The question implies that [[fossil]] [[bird]]s should be discovered with literal "half-wings" &mdash; i.e., a wing missing half of itself. This is a misunderstanding of how wings likely evolved.

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According to the modern theory, evolution is not oriented toward a final goal but merely involves small changes that aid in the survival of individuals to the age of reproduction. In particular, evolution in the ancestor of modern birds was not working toward full-blown modern wings as we know them today. The arms of, say, small dinosaurs might have developed small proto-feathers in order to help them stay in the air longer when they jumped to avoid predators or to pursue prey. This wasn't yet a wing, but it ostensibly had some small benefit to the animal. Over millennia the feathers may have grown longer, the arms leaner and the muscles stronger. Today their ancestors, modern birds, ''do'' have wings as we know them.

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According to the modern theory, evolution is not oriented toward a final goal but merely involves small changes that aid in the survival of individuals to the age of reproduction. In particular, evolution in the [[Wikipedia:Evolution of birds|ancestor of modern birds]] was not working toward full-blown modern wings as we know them today. The arms of, say, small [[wikipedia:Dinosaur|dinosaurs]] might have developed small proto-feathers in order to help them stay in the air longer when they jumped to avoid predators or to pursue prey. This wasn't yet a wing, but it ostensibly had some small benefit to the animal. Over millennia the feathers may have grown longer, the arms leaner and the muscles stronger. Today their descendants, modern birds, ''do'' have wings as we know them.

The "half a wing" argument is related to the idea that evolution is like a [[Tornado argument|tornado moving through a junkyard and assembling a fully-functional 747]].

The "half a wing" argument is related to the idea that evolution is like a [[Tornado argument|tornado moving through a junkyard and assembling a fully-functional 747]].

==What good is half an eye?==

==What good is half an eye?==

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A closely-related question is, "what good is half an eye?" But we all know people who are nearsighted or farsighted, and who get along quite well. Even without glasses, it is better to be nearsighted than to have cataracts, which in turn is better than being blind. "Half an eye" or 50% of vision is 50% better than being blind (well, actually probably much more than 50% better).

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A closely related question is, "What good is half an eye?" We all know people who are [[Wikipedia:Myopia|nearsighted]] or [[Wikipedia:Hyperopia|farsighted]], and who get along quite well. Even without glasses, it is better to be nearsighted than to have [[wikipedia:cataract|cataracts]], which in turn is better than being blind.

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"Half an eye", or 50% of vision, is 50% better than being blind (well, probably much more than 50% better if you consider an animal trying to survive in the wild among sighted predators).

Another way to look at it is to ask what constitutes 100% of an eye, the answer to which can shed light on what 50% of an eye is, and therefore what good it is.

Another way to look at it is to ask what constitutes 100% of an eye, the answer to which can shed light on what 50% of an eye is, and therefore what good it is.

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*The human eye cannot be said to be a full eye in many respects:

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When compared to those of other animals, the human eye cannot be said to be a "full eye" in many respects:

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**The nerves and blood vessels that carry visual signals and blood from the retina to the brain are wired so that they go inside the eye. Where they exit the eye, they crowd out light receptors, resulting in a blind spot that the brain must fill in essentially with educated guesswork. Squid and octopus eyes are not wired this way, and therefore do not have a blind spot.

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*Human vision is based on sensing only [[Wikipedia:Trichromacy|three colors]] (and [[Wikipedia:Color blindness|sometimes less]]), not [[Wikipedia:Pentachromacy|five]] as in some birds and fish.

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**Humans see only three colors, not five as in some fish.

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*Humans cannot see [[wikipedia:ultraviolet|ultraviolet]], as can [[wikipedia:bee|bees]].

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**Humans cannot see ultraviolet like bees.

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*Humans cannot see [[wikipedia:infrared|infrared]], as can [[wikipedia:pit viper|pit vipers]].

*Humans cannot see [[wikipedia:magnetic field|magnetic fields]], as [[Wikipedia:Bird migration#Orientation and navigation|migratory birds]] seem to be able to do.

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**Humans cannot see magnetic fields unlike some migratory birds.

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*Humans cannot detect [[Wikipedia:Polarization (waves)#Biology|polarized light]] unlike bees and [[wikipedia:ant|ant]]s (the ability is most developed in the so-called [[wikipedia:mantis shrimp|mantis shrimp]]).

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**Humans cannot easily detect polarized light unlike ants.

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*Human eyes have a [[wikipedia:Fovea centralis|fovea]], an area densely packed with light receptors, which we use for distinguishing [[wikipedia:color vision|colors]] and for [[Wikipedia:Visual acuity|resolving fine detail]] (this is why we can't read a book out of the corner of our eye: there aren't enough receptors to make out the shapes of the letters). But some birds of prey, such has hawks, have [[Wikipedia:Bird vision#Diurnal birds of prey|two foveas]], which allows them to resolve detail in two areas at once.

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**Human eyes have a fovea, an area densely packed with light receptors, which we use for distinguishing colors and for resolving fine detail (this is why we can't read a book out of the corner of our eye: there aren't enough receptors to make out the shapes of the letters). But some birds of prey, such has hawks, have two foveas, which allows them to resolve detail in two areas at once.

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*Human eyes (along with all vertebrates) have a built-in [[Wikipedia:Blind spot (vision)|blind spot]] where the nerves and blood vessels that carry visual signals and blood from the retina to the brain exit the eye, crowding out light receptors. The brain must fill in the missing information using essentially educated guesswork. [[Wikipedia:Squid|Squid]] and [[wikipedia:octopus|octopus]] eyes are not wired this way and therefore do not have a blind spot.

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**Chameleons have independently-targetable eyes: each eye can look at, and focus on, a separate object. They can estimate the distance to each object by the way the lens focuses on that object.

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*Humans cannot point their eyes in two different directions and focus on two different objects (and [[wikipedia:depth perception|judge the distance]] to each object independently), as can [[wikipedia:chameleon|chameleons]].

Eye evolution is well known and documented from a point of light sensitive cells, to a depression for the light sensitive cells, to a pin hole eye, to a lensed eye: each step conveys more and more use to an organism. So half an eye is useful, about half as useful.

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The sequence of steps necessary to develop a sophisticated eye is well known and each step has been extensively documented in nature, from a simple patch of light-sensitive cells, to a depression for the light-sensitive cells (to sense the direction of the incoming light), to a "pin hole" eye (to focus an image), to a lensed eye (to protect the eye contents and allow more light in): each step conveys additional benefits to an organism. So "half an eye" ''can'' be useful. (For more detail, see [[Wikipedia:Evolution of the eye]].)

==See also==

==See also==

Revision as of 23:56, 22 April 2011

The question, "What good is half a wing?" is often used by creationists who severely misunderstand evolutionary theory. The question implies that fossilbirds should be discovered with literal "half-wings" — i.e., a wing missing half of itself. This is a misunderstanding of how wings likely evolved.

According to the modern theory, evolution is not oriented toward a final goal but merely involves small changes that aid in the survival of individuals to the age of reproduction. In particular, evolution in the ancestor of modern birds was not working toward full-blown modern wings as we know them today. The arms of, say, small dinosaurs might have developed small proto-feathers in order to help them stay in the air longer when they jumped to avoid predators or to pursue prey. This wasn't yet a wing, but it ostensibly had some small benefit to the animal. Over millennia the feathers may have grown longer, the arms leaner and the muscles stronger. Today their descendants, modern birds, do have wings as we know them.

What good is half an eye?

A closely related question is, "What good is half an eye?" We all know people who are nearsighted or farsighted, and who get along quite well. Even without glasses, it is better to be nearsighted than to have cataracts, which in turn is better than being blind.

"Half an eye", or 50% of vision, is 50% better than being blind (well, probably much more than 50% better if you consider an animal trying to survive in the wild among sighted predators).

Another way to look at it is to ask what constitutes 100% of an eye, the answer to which can shed light on what 50% of an eye is, and therefore what good it is.

When compared to those of other animals, the human eye cannot be said to be a "full eye" in many respects:

Human eyes have a fovea, an area densely packed with light receptors, which we use for distinguishing colors and for resolving fine detail (this is why we can't read a book out of the corner of our eye: there aren't enough receptors to make out the shapes of the letters). But some birds of prey, such has hawks, have two foveas, which allows them to resolve detail in two areas at once.

Human eyes (along with all vertebrates) have a built-in blind spot where the nerves and blood vessels that carry visual signals and blood from the retina to the brain exit the eye, crowding out light receptors. The brain must fill in the missing information using essentially educated guesswork. Squid and octopus eyes are not wired this way and therefore do not have a blind spot.

Humans cannot point their eyes in two different directions and focus on two different objects (and judge the distance to each object independently), as can chameleons.

The sequence of steps necessary to develop a sophisticated eye is well known and each step has been extensively documented in nature, from a simple patch of light-sensitive cells, to a depression for the light-sensitive cells (to sense the direction of the incoming light), to a "pin hole" eye (to focus an image), to a lensed eye (to protect the eye contents and allow more light in): each step conveys additional benefits to an organism. So "half an eye" can be useful. (For more detail, see Wikipedia:Evolution of the eye.)